Aluminum-iron alloy for exhaust valves of internal combustion engines

ABSTRACT

IRON-ALUMINUM ALLOYS PARTICULARLY USEFUL IN EXHAUST VALVES FOR INTERNAL COMBUSTION ENGINES, THE ALLOYS HAVING AN IRON-ALUMINUM MATRIX CONTAINING FROM 30 TO 50 ATOMIC PERCENT ALUMINUM, AND ONE OR MORE HARDENING ELEMENTS TOTALING NOT IN EXCESS OF 6 ATOMIC PERCENT.

United States Patent O ALUMINUM-IRON ALLOY FOR EXHAUST VALVES OFINTERNAL COMBUSTION ENGINES John C. Sawyer and Charles F. Barth,Cleveland, Ohio, assignors to TRW Inc., Cleveland, Ohio No Drawing.Filed Nov. 17, 1967, Ser. No. 683,816 Int. Cl. C22c 37/10 US. Cl. 75-1241 Claim ABSTRACT OF THE DISCLOSURE Iron-aluminum alloys particularlyuseful in exhaust valves for internal combustion engines, the alloyshaving an iron-aluminum matrix containing from 30 to 50 atomic percentaluminum, and one or more hardening elements totaling not in excess of 6atomic percent.

BACKGROUND OF THE INVENTION Field of the invention This invention isdirected to an improved alloy system having excellent strengthproperties at elevated temperatures and improved lead oxide corrosionresistance, the alloy having a matrix consisting both of iron andaluminum having sufficient aluminum present to favor the production ofthe intermetallic compound FeAl, and containing one or more hardeningelements in amounts totaling not in excess of about 6 atomic percent,the hardening elements being such materials as zirconium, molybdenum,carbon, titanium, vanadium tantalum and lanthanum.

Description of the prior art There are numerous criteria involved in thechoice of suitable materials for exhaust valves. These have been wellstated in the chapter entitled The Selection of Material for EngineValves prepared by the American Society for Metals Committee on EngineValves, appearing at pages 626 to 634, inclusive, of Metals Handbook,Eighth edition, volume 1. As detailed in that article, typical wroughtand cast alloys used for valves may include ferritic steels, sigma phasealloys, austenitic alloys or nickel alloys. For the most part, however,these alloys contain no aluminum or those that do contain very smallamounts.

SUMMARY OF THE INVENTION The present invention provides an iron-aluminummatrix useful for exhaust valves in internal combustion engines becauseof its low density, excellent corrosion resistance, hot hardness,mechanical strength, and low cost. The matrix consists of aniron-aluminum binary alloy containing from 30 to 50 atomic percentaluminum (17.1 to 32.6 weight percent), the amount of aluminum presentbeing conducive to the formation of the intermetallic compound FeAl.Studies have indicated that the oxides formed at elevated temperatureson alloys of this type have a relatively high resistance to lead oxideattack. However, alloys having high aluminum contents are frequentlybrittle and lack the hot hardness and mechanical strength required foruse in valves operating at temperatures of 1200 to 1600 F. Theseproblems have now been satisfactorily met by control of processingvariables and by the addition of suitable hardening elements incontrolled amounts, the hardening elements being one or more of the c CCfollowing group: zirconium, molybdenum, carbon, titanium, vanadium,tantalum and lanthanum. We have found that when these hardening elementsare added in amounts totaling not in excess of about 6 atomic percent,the hot hardness and mechanical strength properties of the ironaluminummatrix are improved to the extent that they are completely useful inexhaust valves operating at elevated temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The alloy of the presentinvention, in its broader aspects, contains from 50 to atomic percentiron, from 30 to 50 atomic percent aluminum, and one or more hardeningelements, these hardening elements including up to 1 atomic percent.zirconium, up to 1.5 atomic percent carbon, up to 5 atomic percentlanthanum, up to 5 atomic percent vanadium, up to 5 atomic percenttantalum, up to 5 atomic percent molybdenum and up to 5 atomic percenttitanium. In the preferred form of the invention, the matrix containsfrom 38 to 42 atomic percent aluminum, the balance being iron, and thehardening elements include the addition of from 0.1 to 1.0 atomicpercentzirconium, from 0.05 to 1.0 atomic percent carbon, from 1 to 4 atomicpercent vanadium, from 1 to 4 atomic percent tantalum, from 1 to 4atomic percent molybdenum, from 0.1 to 4 atomic percent titanium andfrom 0.1 to 4 atomic percent lanthanum.

The alloys of the present invention have yield strengths of at least30,000 p.s.i. at 1400 F., and a lead oxide corrosion loss not in excessof 25 grams per square decimeter per hour at 1675 F. The lead oxidecorrosion test is a laboratory bench-type test employed for evaluationof corrosion resistance of exhaust valve alloys, and involves preparinga specimen of the test alloy, 0.444 inch in diameter by 0.444 inch inlength and placing this specimen in a magnesia crucible with 40 grams oflead oxide, and heating to 1675 F. for one hour. The specimen is thencooled to room temperature, cleaned and weighed. The loss in Weightexpressed per unit area represents a measure of the relative corrosionresistance of the alloy.

The strengthening of the iron-aluminum matrix can be attained throughalloy additions of either molybdenum, tantalum or vanadium or throughfurther additions based on titanium, carbon and zirconium in combinationwith the preceding additives. These additives appear to provide improvedage hardening properties to the alloys. The following table illustratesthe results obtained when varying amounts of additives were added to aniron-aluminum matrix containing 40 atomic percent aluminum:

TABLE I Brlnell Hardness 3,000 kg. load 750 k load Alloy Atomic Weight10 mm. ball: a 1113i. ball: addition percent percent room temp. 1,400 F.

The mechanical properties of some of the alloy compositions, showing theimprovements achieved by the var ous alloying additions are summarizedin the following table:

4 The best overall properties achieved in alloys produced according tothe present invention occur in those alloys TABLE II Mechanicalproperties at 1,400 E, 0.2% ofiset Yield Tensile Percent Nominalcomposition, strength, strength elon Heat No. atomic percent Condition Ks 1 K s.i. gation As extruded 21 23 80 3 F2381 "{Aged 100 hr. at 1,60019 23 122 9 Fe-46Al-1Ti-0.2Zr-0.2C is exrugeg S 8X 1'11 8 9A "{Solutiontrgated 2 hrs. at 2,000 F., oil quenched. As extrude {Solution treated 2hrs. at 2,00o F., oil quencheda5, 2 47 2 47. 2 40, 2 so F 411 2M 12 Asextruded 27 35 74 11 e Solution treated 2 hrs. at 2,000 F., oilquenched. 31 39 80 1 Premature fracture.

2 Specimen tested at the normally used crosshead speed of 0.1 inch perminute, the other data were obtained at a slower crosshead speed of 0.01inch per minute.

The hot hardness of the alloys was evaluated using samples /2 inch indiameter by /2 inch long. The samples were prepared for hot hardnessevaluation in the as extruded condition and after a stabilizationtreatment at 1800 F. for 16 hours. As evident from the following table,the stabilization treatment increases the hot hardness of the extrudedalloy by approximately in most instances.

TAB LE III Brinell Hardness 750 kg. load Nominal composition, 1,400 F. 5mm. ball, Heat No atomic percent as extruded stabilized 1 Fe-40Al-1M0 823 98 4 Z 5 104 6 103 7 101 Several samples each of the various alloyswere subjected to 1675 F. lead oxide corrosion test in the as extrudedcondition. The average values attained in the lead oxide corrosion testare given in the following table:

which contain the combination of molybdenum, titanium, carbon, andzirconium. These alloys have yield strengths of about 47,000 p.s.i. at1400 F., and a lead oxide corrosion resistance of 11 to 12 grams persquare decirneter per hour at 1675" F.

The relatively low cost of the alloys of the present invention makes itpossible to produce the entire valve structure from the alloys and stillremain competitive with valve alloys containing substantial amounts ofchromium. If desired, however, the application of the alloy can berestricted to facing of the valve seat or other portions of the valvewhich are subjected to high temperatures and severe lead oxide corrosionproblems.

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

We claim as our invention:

1. An exhaust valve for an internal combustion engine composed at leastin part of an iron-aluminum matrix consisting essentially of from 38 to42 atomic percent aluminum, balance iron, and at least one hardeningelement selected from the group consisting of 0.1 to 1.0 atomic percentzirconium, 0.05 to 1.0 atomic percent carbon, 1 to 4 atomic percentvanadium, 1 to 4 atomic percent tantalum, 1 to 4 atomic percentmolybdenum, 0.1 to 4 atomic percent titanium, and 0.1 to 4 atomicpercent lanthanum.

References Cited UNITED STATES PATENTS 2,579,369 12/1951 Dawe 1242,604,455 7/1952 Reynolds 75124 2,946,676 7/ 1960 Brennan 751243,303,561 2/1967 Cabane 75124 3,386,819 1/1968 Cabane 75124 3,420,660 1/1969 Kawahatta 75124 HYLAND BIZOT, Primary Examiner US. Cl. X.R.

